1. Field of the Invention
This invention is concerned with an air gun having a reciprocating shuttle valve for use in marine seismic exploration such as will be found in class 181/120.
2. Description of the Prior Art
The subject matter of this invention provides an improvement over the air guns disclosed in U.S. Pat. No. 4,180,139, 4,211,300 and applications Ser. No. 180,788 now U.S. Pat. No. 4,324,311, and Ser. No. 289,564, patent pending all of which are commonly owned by a sister firm of the assignee of this invention. Application Ser. No. 180,788, now U.S. Pat. No. 4,324,311 is incorporated herein by reference with respect to features that are common to that patent and to this disclosure.
The referenced patent discloses an air gun that consists of an elongated cylindrical housing, closed at both ends by end plates, for containing a volume of compressed air and which forms a firing chamber. Radially-positioned air exhaust ports are bored through the wall of the housing. A hollow cylindrical shuttle valve is mounted concentrically inside the housing. The shuttle valve is adapted to slide linearly back and forth, inside the housing, in alternate strokes. One or more radially-positioned sealing pads, having orifices that are alignable with the exhaust ports, are spring-mounted in apertures in the wall of the shuttle. When the shuttle is positioned at either end of the housing, the pads block the exhaust ports. When the shuttle is driven from one end of the housing to the other end, the orifices in the pads pass by the exhaust ports and momentarily unblocks the exhaust ports to allow impulsive release of some of the compressed air from the firing chamber.
In the practical design of the air gun of the referenced patent, several sealing pads were floatingly mounted in a corresponding number of relatively large apertures in the shuttle valve body. Small springs between the sealing pads and the shuttle body, held the sealing pads firmly against the inner wall of outer housing when the air gun was unpressurized. When the gun was disassembled for maintenance, the floating pads and springs had a disconcerting tendency to fall out of the apertures. Further, upon reassembly, the several pads were difficult to hold in place, all at the same time, while sliding the shuttle back into the housing. Other problems, including air leakage between the sealing pads and the inner housing wall made it necessary to provide an improved sealing member or valve.
An additional problem arose in connection with the relatively large apertures that were cut into the shuttle body to receive the sealing pads. Because the shuttle is forcibly driven against the stops at opposite ends of the gun, it was subjected to repeated severe shock loadings. The presence of the large apertures structurally weakened the shuttle body and fatigue fractures occurred at the corners of the apertures. But making the shuttle more massive to compensate for the inherent structural weakness, added more weight. Additional shuttle weight of course, demanded more power to overcome shuttle inertia.